TDI is an important material for producing polyurethane, which is mainly used as the material and intermediate for polyurethane soft foams, rigid foams, adhesives, coatings, sealants and a series of elastomers. TDA, as an essential material for preparing TDI and an important material for preparing a variety of dyes and medical intermediates, also has a broad market prospect.
At the end of the synthesis process of TDI, TDI is usually isolated from the product mixture by means of a rectification method. High boiling point solid residues or distillation remains discharged from the fraction at the bottom of the rectification column upon drying are generally called tar powder. The tar powder is a mixture consisting of polybiuret, byproducts and a plurality of impurities.
The prior art has described various methods of directly utilizing the substances in the distillation remains discharged from the synthesis process of TDI. In U.S. Pat. No. 3,499,021, the distillation remains are subjected to phosgenation treatment and returned to the process. In DE4211774, DD257827 and U.S. Pat. No. 3,694,323, the distillation remains are mixed with diphenylmethane diisocyanate (MDI), and partially distilled and transformed into polyurethane. DD296088, U.S. Pat. No. 4,143,008, U.S. Pat. No. 4,000,099 and U.S. Pat. No. 4,311,800 describe that the distillation remains are directly reacted with polyols to form corresponding polyurethane for the preparation of synthetic resins, lignocelluloses, and the like. However, the additional value of the products obtained by these processes is not high, and they consume a large amount of alcohols (the alcoholysis reaction requires to consume at least 24% of polyols), and have a high cost.
Another way to utilize the distillation remains is to hydrolyze them. It is a well-known technique to hydrolyze TDI tar powder by using the alkaline aqueous solution added with ammonia water and alkaline earth hydroxide, or the acid aqueous solution of inorganic and organic acids so as to prepare TDA. However, the tar powder has special physiochemical properties and is hardly soluble in both water and organic solvents, the hydrolysis thereof belongs to non-homogeneous reactions, and the process of transferring mass and heat is slow, thus the speed of the hydrolysis reaction under normal temperature and pressure is extremely slow and the yield thereof is very low.
Although the yield of TDA can be increased by the methods of increasing the hydrolysis temperature and pressure and improving the way to mix and contact the materials, the extent of the yield increase is limited due to the restrictions by the low boiling point of water and poor water solubility of the reactants, thus the practicability is quite poor. The reports in this respect can be found in U.S. Pat. No. 3,331,876, U.S. Pat. No. 3,128,310, DE2942678, DE1962598 and JP58-201751. In addition, the hydrolysis technique introduced by the Korean patent 2001-52948 adopts a continuous or semi-continuous reverse-mixing reactor, which improves the efficiency of mass transfer to some extent, but just increases the degree of mechanical mixing instead of improving the activities of the reactants. It is still difficult to get rid of the restriction by the resistance to mass transfer and thus has little effect.
Use of subcritical water or supercritical water hydrolysis may further improve the hydrolysis temperature and pressure, speed up the reaction, and increase the yield of TDA. The Korean patent 2001-1488 discloses a method in which ammonia water is used as a catalyst for the hydrolysis of tar waste materials and the hydrolysis is conducted in supercritical water at 350-600° C. under 218-400 atmospheric pressures. Although the high temperature and pressure conditions of the supercritical water can speed up the hydrolysis, the supercritical water not only causes the device to corrode, but also decreases the solubility of various salts; and moreover, ammonium hydrocarbonate, ammonium carbonate and organic polyamine salt as well as composite hydrate salts transformed from partial ammonia water during the hydrolysis process would all cause the problems such as blocking in pipelines of the device and secondary pollution in the environment.
The Chinese patent CN200480015939.X makes a series of improvements to the supercritical hydrolysis of tar waste materials, the hydrolysis product of toluene diisocyanate (TDA) has a notably-increased yield and can be returned back to the TDI production process, and its hydrolysis catalyst of carbonate and water can be recycled by means of the recovery. Said invention selects alkaline earth hydroxide or carbonate as a catalyst, which can avoid the problems brought by the catalyst of ammonia water, but still does not solve the problems of poor solubility of the reactants in water and difficulty in transferring mass, thus the invention still adopts the supercritical hydrolysis under high temperature and pressure (100-200 atmospheric pressures, temperature of 280-320° C.). Accordingly, it is difficult to get rid of the problems of high requirement and investment on the device, production security being difficult to guarantee, and blocking in pipelines of the device brought therefrom, thus the industrialization is hard to realize.
Based on the above problems, the preparation of TDA from TDI distillation remains are not implemented at an industrial-scale yet, thereby resulting in that most of the TDI distillation remains currently have to be burned at a high temperature, which not only causes waste in a large amount of resources but also can hardly avoid secondary pollution.
The treatment of tar powder with a hydrolysis method can greatly reduce the amount of TDI distillation remains to be burned, can recover and utilize TDA with a high additional value from the waste, thereby achieves the purpose of energy saving, emission reduction and resource recycling in the isocyanate industry, however, the preparation of TDA by hydrolyzing TDI tar waste residues belongs to non-homogeneous reactions, the hydrolysis at a temperature lower than 250° C. would lead to the deficiencies in large resistance to mass transfer and poor activity of the reaction, while the hydrolysis at a temperature higher than 400° C. would cause the problem that the target product TDA is easily pyrolyzed. This results in that the existing methods involve the following problems: it is difficult to raise the temperature for the hydrolysis reaction due to the restriction by the low boiling point of the hydrolysis medium, the efficiency of the hydrolysis is poor; choosing an inappropriate catalyst causes the tar powder to transform into organic salts with a low value, recycling of resources cannot be realized; using an excess amount of water, waste water or nitrogen components contained in discarded substances causes secondary pollution; the method under high temperature and pressure may increase the hydrolysis yield to some extent, however, the investment in the device for supercritical hydrolysis is high, and the operation security thereof cannot be guaranteed, it is hard to realize real industrialized production; the alcoholysis process consumes a large amount of alcohols, the target product does not have a high value; the yield of TDA is extremely low when it is prepared through the hydrolysis after the alcoholysis, the economic applicability is poor. Consequently, direct preparation of TDA by hydrolyzing TDI process tar waste residues is quite difficult, the prior art can hardly meet the requirement of the industrialization.